1. Field of the Invention
The present invention relates to a flat-shaped nonaqueous electrolyte battery such as a button-type nonaqueous electrolyte battery and a coin-type nonaqueous electrolyte battery. More specifically, it relates to a flat-shaped nonaqueous electrolyte battery that has a structure in which a sealing plate also serves as a positive electrode terminal and a metal container also serves as a negative electrode terminal, and achieves large capacity and excellent cycle characteristics as well as high reliability.
2. Description of Related Art
For both primary batteries and secondary batteries, nonaqueous electrolyte batteries having a flat shape such as a button shape and a coin shape are used widely as a main power source and a memory backup power source in various electronic appliances.
With a miniaturization of electronic appliances, the demand has shifted recently from conventional batteries having a diameter of about 10 to 20 mm to smaller batteries having a diameter of 8 mm or less.
Conventionally, in such a flat-shaped nonaqueous electrolyte battery, as shown in FIG. 5, an edge portion of an opening of a metal container 4 serving as a positive electrode terminal tightly is pressed inward, so that the metal container 4, a sealing plate 5 serving as a negative electrode terminal and a gasket 6 seal in electric-power generating elements such as a negative electrode 1, a positive electrode 2 and a nonaqueous electrolytic solution obtained by dissolving a lithium salt in an organic solvent. Usually, the negative electrode 1 and the positive electrode 2 have substantially the same area, or the negative electrode 1 has an area smaller than the positive electrode 2 because the negative electrode 1 is arranged on the side of the sealing plate 5.
In the above-described flat-shaped nonaqueous electrolyte battery, the capacity of the battery is designed to be dependent mainly on the capacity of the positive electrode, while cycle characteristics of the battery is dependent mainly on the capacity of the negative electrode. Accordingly, the capacity and cycle characteristics of the battery are determined by a balance between the capacity of the positive electrode and that of the negative electrode. In conventional batteries with a diameter of about 10 to 20 mm, the capacity ratio of the negative electrode and the positive electrode is determined under the condition that both the electrodes have substantially the same area as described above, and these batteries have been designed such that the capacity and cycle characteristics of the battery both are satisfactory to a certain extent.
However, in conventional batteries having a relatively large diameter, there also has been a demand for still larger capacity while maintaining excellent cycle characteristics. Moreover, in the case of smaller batteries, when the size of the flat-shaped nonaqueous electrolyte battery with the above conventional structure is reduced, the volume ratio of elements other than the electric-power generating elements such as the gasket increases with respect to the entire battery. Accordingly, the decreasing rate of the effective volume inside the battery is larger than that of the volume inside the battery, causing a problem that a battery with a desired capacity cannot be designed. Furthermore, accompanying the miniaturization of batteries, the content of electrolytic solution is restricted. Thus, the cycle characteristics of the batteries have become further dependent on the capacity ratio of the positive electrode and the negative electrode, leading to a problem in that the adoption of the capacity ratio as in the conventional battery with a diameter of about 10 to 20 mm deteriorates the cycle characteristics considerably.
Attempts to reconcile these concerns have met with mixed success. The capacity ratio of the negative electrode may be raised by reducing the positive electrode ratio. However, the capacity of the battery decreases considerably, leading to a battery of little practical use. When the amount of the electrolytic solution is raised, the cycle characteristics improve but the electrolytic solution leaks easily. When the volume ratio of the gasket is reduced in order to secure the effective volume inside the battery, a problem arises in that a sealing performance by the gasket declines, leading to poor sealing characteristics.
In one aspect, the present invention provides a flat-shaped nonaqueous electrolyte battery that achieves large capacity, excellent cycle characteristics and leakproof characteristics as well as high reliability.
In one embodiment, the present invention has the following structure: a flat-shaped nonaqueous electrolyte battery includes a sealing plate, a metal container, a gasket interposed between the sealing plate and the metal container, and a negative electrode, a positive electrode and a nonaqueous electrolyte that are sealed by pressing inward an edge portion of an opening of the metal container. The sealing plate also serves as a positive electrode terminal, the metal container also serves as a negative electrode terminal, and a peripheral portion of the negative electrode is arranged between an inner bottom surface of the metal container and the gasket. With this structure, a flat-shaped nonaqueous electrolyte battery that achieves large capacity, excellent cycle characteristics and leakproof characteristics as well as high reliability is provided, thereby solving the above-mentioned problems.
Because the effect of the battery structure of the present invention increases with a decrease in a battery diameter, such a structure is used preferably in a battery having a diameter of 2 to 16 mm. In particular, the effect is significant in a small battery having a diameter of 2 to 8 mm.